LEDs are becoming more widely used as light sources, and are attractive owing to their long lifetime and low power consumption. A number of LEDs and a driving circuit can be incorporated in a “retrofit” light bulb, for example to replace a less energy-efficient incandescent light bulb. An incandescent lamp can be driven directly from an AC mains voltage, for example 240 V. A halogen lamp is generally driven from a transformer which lowers the input voltage level. These types of power supply are not suitable for an LED, which requires a relatively low DC voltage to operate safely. There are various ways of using LEDs with existing power supplies. For example, multiple LEDs can be used in a configuration to match the high input voltage. Alternatively, driver electronics can be placed between the power supply and the LED light source to convert the input voltage and current to suitable and safe levels.
An LED load will emit light when the voltage across the LED load exceeds a certain minimum. As the voltage increases, the LED current increases accordingly, resulting in more light output. Generally, the input voltage should not exceed a rated voltage level, since excessively high LED current ultimately decreases the lifetime of the LED. For these reasons, it is important that the voltage across an LED load is maintained at or below the rated level, and that the LED current does not exceed a recommended drive level. This can be achieved in a number of ways. In one approach, the LED current can be controlled in a relatively straightforward manner using a semiconductor switch such as a bipolar junction transistor (BJT) in series with the LED load, and by applying an appropriate voltage to the base of the BJT to regulate the amount of current passed by the switch. A current control circuit regulates the BJT base voltage to ensure that the LED current remains constant over the input voltage range between the minimum level and the rated level. When the input voltage exceeds the rated voltage, LED current remains constant, and the excess power is dissipated by the current control circuit. As a result, this control approach is associated with unfavourably high losses.
In configurations comprising a variable number of LED lamps in parallel for example in a lighting fixture with a parallel arrangement of retrofit LED bulbs the power supply must deliver a constant voltage, but current regulation must be done at each individual LED lamp. In order to control the light output of the LED lamps, i.e. to increase or decrease their light output as desired, usually the supply voltage is the only available control parameter, so that voltage controlled dimming is the technique generally used. However, in the case of LED loads, the desire to limit the driver losses conflicts with the control range and linearity of the LED lighting arrangement.
Therefore, it is an object of the invention to provide an alternative way of regulating the current through an LED load.